FM has a technical limit of 8TB per file. You'd only be at risk of hitting that if you were to embed all the container data inside the FM file, and that's not recommended by anyone.

So you could use the FM remote container feature to keep everything efficient. If you already have embedded data in containers it is as simple as changing the type of the container from embedded to remote and FM will take care of extracting and saving the data for you. May take a long time if you have a lot of data.

With FM's remote container feature, keep in mind though that all access to that document data has to flow through FM, you should not touch that document data outside of FM. If you need to be able to do that then FM's remote container feature is not for you.

Personally, with that amount of data I would look for an off-the-shelf document management system that has good APIs, then use FM to work with those APIs so that FM can do what really brings value and let the document management to a system that is already built for it.

You can also get good results with server co-location where you supply the hardware. There are also companies out there that will provide that level of storage in a professional hosting environment for a reasonable cost.

Wim has good ideas. But if you want something that big in the cloud, you need to get your own instance so you are running your own unshared server. It could be from any provider from Amazon to Rack Space or wherever. The storage and the speed with which the computer can access that much storage is a big issue too.

If you are going to use FileMaker for document management, please make sure to use the default secure external storage. While the security is important, the main reason to do this is that FileMaker manages the folders and files per folder which can effect performance a lot. I've solutions try to manage this on its own and it becomes a big problem, especially ones that try to put everything in one folder!

If you need anything other than FM to access the same documents, then using a Document Management system with good RESTful API could be integrated well with FileMaker as well as other services needing the same access.

And lastly, your plans for backups become a big issue when you have that much data. Not sure what you are doing, but you have to consider that too.